1. Field of the Invention
This invention relates to a process for the production of polycarboxylates by selective oxidation of polysaccharides with nitrogen dioxide and to the use of the polycarboxylates obtained by this process as builders or co-builders in detergents or cleaning preparations.
2. Discussion of Related Art
Extensive knowledge exists on the production of polycarboxylates by oxidative treatment of polysaccharides, for example cellulose, starch and dextrins, cf. for example Houben-Weyl "Methoden der organischen Chemie", Thieme-Verlag, Stuttgart (1987), Vol. E 20, Makromolekulare Stoffe, subchapter entitled "Polysaccharid-Derivate" edited by Dr. K. Engelskirchen, loc. cit., pages 2042 et seq., more particularly pages 2124 et seq. (oxidation products of cellulose) and pages 2166 et seq. (oxidized starches), and the publication entitled "Cellulose Chemistry and its Applications,, (1983), John Wiley & Sons, Chichester, GB, more particularly chapter 10 "Oxidation of Cellulose" by T. P. Nevell and the extensive literature cited therein, loc. cit., pages 262 to 265.
Roughly summarized, it may be said that several oxidizing agents are commonly used for the oxidation of polysaccharides, more particularly polyglucosans produced exclusively from glucose. They include, for example, (atmospheric) oxygen, hydrogen peroxide, sodium chlorite or bromite, periodic acid and periodates, lead(IV) acetate, nitrogen dioxide and cerium(IV) salts. These oxidizing agents react very differently with the anhydroglucose units, cf. for example the formula schemes in Houben-Weyl, loc. cit., page 2124. For example, periodates or lead(IV) acetate promote C--C cleavage of the anhydroglucose rings; so-called 2,3-dialdehyde cellulose is obtained from cellulose and dialdehyde starch is similarly obtained from starch. It is also known that, when cellulose is exposed to the action of nitrogen dioxide, oxidation of the primary alcohol group to the carboxyl group is by far the predominant reaction. The oxidizing agent, generally present in equilibrium with dinitrogen tetroxide, may be used in gaseous form or in solution in an inert organic solvent, cf. Houben-Weyl loc. cit., page 2125 and the primary literature cited in this connection therein. It is even possible starting out from starch to achieve substantially selective oxidations of the primary alcohol group of the anhydroglucose units to the carboxyl group. Thus, the oxidation of starch with gaseous nitrogen dioxide or nitrogen dioxide dissolved in water or in various organic solvents at room temperature/normal pressure is known from U.S. Pat. No. 2,472,590.
Under these conditions, the substantially complete conversion of the primary alcohol groups of the polysaccharides into carboxyl groups is only achieved after very long reaction times which can amount to several days. In addition, large amounts of nitrogen dioxide, based on the polysaccharide to be oxidized, are required in the known process.
The problem addressed by the present invention was to enable the production of such oxidation products of polysaccharides to be improved to secure their availability because the polycarboxylates formed are of interest as potential ingredients, more particularly as builders or co-builders, of detergents and cleaning preparations. The same also applies to the salts of such polycarboxylates, more particularly their water-soluble salts, because the use of oxidized polysaccharide compounds to boost the performance of detergents and/or cleaning preparations has basically been known and repeatedly investigated for decades, cf. for example Dutch patent applications NL 69 883 and NL 78 087. The replacement of phosphate-based builder systems by 6-carboxycellulose treated with Lewis acids is described in U.S. Pat. Nos. 3,740,339 and 3,790,561. Dutch patent application NL 70/02 500 also seeks to use oxidized polysaccharide derivatives as builder systems for increasing detergent performance, particularly in laundry detergents. However, this document is not concerned with derivatives selectively oxidized at the C.sub.6 atom, but instead with oxidation products formed by substantial cleavage of the anhydroglucose units between C.sub.2 and C.sub.3. Finally, European patent application EP 425 369 describes surfactant-containing laundry detergent mixtures containing a builder system of conventional phosphate compound, zeolite and oxidation products of cellulose, starch or glucose sirup. There are no reproducible disclosures relating to the preparation of the oxidized saccharide compounds described therein. In addition, stabilization of the polysaccharide oxidates initially formed by catalytic hydrogenation is described as desirable.
The teaching according to the invention is based on the surprising observation that polycarboxylates can be inexpensively obtained in high yields from polysaccharides by a simple process in which the oxidation reaction is carried out with nitrogen dioxide/dinitrogen tetroxide in the presence of oxygen at elevated temperatures. The expression "nitrogen dioxide/dinitrogen tetroxide" stands for the equilibrium mixture of nitrogen dioxide and its dimer, dinitrogen tetroxide, present under the particular reaction conditions.